Seals are used between inner and outer wellhead tubular members to contain internal well pressure. The inner wellhead member may be a casing hanger located in a wellhead housing and that supports a string of casing extending into the well. A seal or packoff seals between the casing hanger and the wellhead housing. Alternatively, the inner wellhead member could be a tubing hanger that supports a string of tubing extending into the well for the flow of production fluid. The tubing hanger lands in an outer wellhead member, which may be a wellhead housing, a Christmas tree, or a tubing head. A packoff or seal seals between the tubing hanger and the outer wellhead member.
A variety of seals located between the inner and outer wellhead members have been employed in the prior art. Prior art seals include elastomeric and partially metal and elastomeric rings. Prior art seal rings made entirely of metal for forming metal-to-metal seals (“MS”) are also employed. The seals may be set by a running tool, or they may be set in response to the weight of the string of casing or tubing. One type of prior art metal-to-metal seal has seal body with inner and outer walls separated by a cylindrical slot, forming a “U” shape. An energizing ring is pushed into the slot in the seal to deform the inner and outer walls apart into sealing engagement with the inner and outer wellhead members, which may have wickers formed thereon. The energizing ring is typically a solid wedge-shaped member. The deformation of the seal's inner and outer walls exceeds the yield strength of the material of the seal ring, making the deformation permanent.
During setting of the seal, the imparted forces may cause a seal leg to deflect downwards relative to the other seal leg. This can introduce plastic strain into the seal, making it susceptible to tear or shear when the casing hanger moves. To address this problem, a threaded connection has been utilized below the seal that connects a nose ring to the seal. The nose ring has a thin, annular tab, that protrudes upward and contacts the inner seal leg. This tab is supposed to resist the setting forces imparted to it when the energizing ring is driven into the seal to thereby prevent the inducement of plastic strain due to inner seal leg deflection.
This same tab is also designed to buckle during pressure testing of the seal and/or BOP stack with a plug-type or isolation tool. During pressure testing a large force, up to several million pounds, is transferred to the top of the casing hanger. This force causes the casing hanger to deflect downwards, carrying with it the inner seal leg, which is engaged to it. At this point the tab is supposed to buckle, allowing independent movement of the inner and outer seal legs. If the legs were rigidly coupled to each other, the seal body would be torn in half from the large load and deflections created by the pressure test. Even with a buckling tab, eventually the relative displacements between the inner and outer seal legs may become so great that the seal will shear itself apart. To limit this relative displacement, test pressures may be lowered, complex load mechanisms on each hanger position may be added instead of a simple stacking arrangement, or wickers may be entirely abandoned on the casing hanger side of the seal in a “slick neck” arrangement. These approaches compromise the robustness of the system.
The annular tab, however, may buckle prematurely due to Poisson effect, which is the tendency of a material to expand in directions perpendicular to the applied compression. In practical applications, the large radial interference between the energizing ring and each of the seal legs causes the seal legs to grow downwards due to the Poisson effect. Because a large radial force is required to effect a gas-tight seal to high pressures, the resulting axial force due to the growth of the seal legs is also high and sufficient to cause the tab to buckle. This premature buckling of the tab may result in a crooked or twisted installation of the seal body and increased plastic strains in the area that MS-type seals typically fail due to excessive hanger movement during pressure testing. To deal with this type of problem, an active hanger with complex mechanisms in the third position could be used. This option however is costly and complex.
A need exists for a technique that addresses the seal problems described above. In particular, a need exists for a technique to make seals more tolerant to increased hanger movement by accounting for Poisson effect in the seal legs. The following technique may solve these problems.